Magnetic focusing structure for electron beams



June 15, 1954 R B, GETHMANN 2,681,421

MAGNETIC FOCUSING STRUCTURE FOR ELECTRON BEAMS Filed March 4, 1952Fig.1.

IIII/IIIIIIIA i 50 Inventor: 5/ Richard B. Gethrhann Sb ZZMZJ D HisAttorn'eg.

Patented June 15, 1954 T.MAGNETIC"TOCUSINGISTRUCTUREFOR ""ELECTRONBEAMS.sRichard :BmGethmann, Eayettevilla/N. :Y.,-. assignor: to. GeneralElectric. "Company;v a lcorporation-of New York Ap'plicatiolFMar'ch 4,1952-;S'erial No; 274,785

12 Claims. 1

1: @My; invention 2 relates :1 to improved; *magnetic focusing.structuressziorc cathode raywitubes and particularly to. such structuresremployingipermaanent izrmagnets. as a 5 source .of :magnetomotive.Lforce.

:rInvcathode ray tubeszof ztheirtypef. presentlyiin large :9 scale;production; focusing7 of thexbeamis zaccom'plishedzby meansmof a.focusing coilz" p'osinction'ed': exteriorlm on; themeck': oilthercathode ray Ietube. This .deviceiisxirather :bulkypexpensive and xrequires-z a..1arge.:amount"-. of magnetic :material. .ISomeIattemptsthave beenwmade in thezprior. art :to accomplish .:the focusingby means of-s a -.magenetic structure supported within :the cathode :raytube envelope. ."L'heseiz prior art structures .have .notfound-commercialsuse .andahave notv rovided ;..the quality. roifocusing.necessary for usecin tubes nxfor television receivers,: for; example.

The l :present".v invention is directed to1an' im- ;proved.internalxfocusing structure; particularly a: oneemployingx-permanentrmagnetsewhich provides. asymmetricalifocusingfieldabout the axis .::of. :the focusing-istru'cture; andwhichzialso providesfora:controlled.predetermined!distribution of the magnetom'otive-fiorcealong .the. axis :of. the .:focusing: structure: .;to provide focusing'. which is v.1substantially free; of a spherical1aberrati'on,..-.or..:in .:many :11. cases 2-. :ivhich provides '...apredetermined 1 amount:oi.Laberration.bfithetproper .sigmto nom-5tpensate'for:aberration-aintroducedrinto the focus- 30*accompanyingtdrawing :zand: tszzscop s gint 'du outiniiutheSappendedtzclaims :the dra'wingiFigi 1 illustrates: an:electron; gun for; a -'rcathodelraytube.incorporating Ia:magneticiwfocus- 1;. ing.structureiembodying myunvention."

-asectionalslviewntaken .alongthedine' 2 1. 3isiamenlargedl'elevationaliwiewzin:section or th'e' magnetid'focusingstructurefiofi Figii 1.

. 2 Fig: 4:.is an elevational zviewvpartia-lly in; section 1' bfaimodifi'ed. magnetic focusing structuretem- :bodyingmyinvention, Fig. 5isa sectional .view .utaken along the line. 5-5 of Fig. 4,; Fig; fiuisan 5 mnelevational .view. in section of a further modificationaiof.:myinvention.and Fig. 7 is anxelevational nviewzin sectionandxpartiallybroken awaymfistill a further modification of my invention.

aReferringe now :to- Fig. 1;. Lhave :shown :my inventionziembodied yinaxcathode: ray= itubezrgun rstructure" supported withinlthe. neck I nota::cath- -r.ode 'ray .tube envelope. including a zstem member .12.from::which thesgun isxsupportedand through twhich1..the.xlead;-inconductors: 3 are; sealed. .azln :1; accordance with :usual' practice..these .aleadein conductors provide externally accessible connections tothe various elements of the: gun and. also support the gun r structurewithin ..the s envelope. LJFDhe gunillustrated is of the :tetrode typeand in-- :cludes. a. grid :member. 4 formed; asanz-zinvertedx;cupihavingwarraperture. 5.;throughsawhich elec- -;trons pass. The.electrons are supplied. Joya. suit- ;zzable. .cathode 5,supportedvwithin the: grid: cylinder with the emittingsurfaceiadjacenmthe.grid aperturerin :a' manner vwell. understood. by?those iskilledairrthe. :art. :second or accelerating; :grid ..\1'lxof.".cuppshapeyandw having-r a *beamqaperturei 8 a. aissupported.intinverted relation with respectlto thegig'ridlcylinder lrbyzmeans of.a:::plurality :.of glass. stalks 9.secured.to:the grid cylinders Land Iby means of radially. extending .pins 10.. The gun .=:strncture. also:includes a cylindrical csecond.- grid ssror anode member-I I. supportedinwaxiallypspaced -.1".elation:with. respect to the accelerating rid 1.1;by radially .-extending..pins:l2.

' The.magneticl.;focusing structure ofu thecem- ;.bodimentiillustratedFigs. Land 3;isisupported .:;direct1y :fromthe.finalpanodei cylinder .ll; which axis made: of stainless steel orsother essentially .non-

amagnetic-material. in'lihermagnetic lens-:=:member *rsiSJiIlJlZh :iormnot a machined cylinderJ 3= secured wwithin 'the .z.;anode .1 cylinderII. As..:;illustr.ated ziclearly in Fig;' 3', cylinder I l gincludes,.polepieces WI 5' :and-d 5 provided by annularend.portionswhich endportions just-ufitithe. inner."diameter.;ofi'.;;the

r finalanode tl hand which maybe. secured in posirstiongby welding tothe anode. sleeve; .The iocusing a m-member! 31: is apertured asshown-ate! BFtOIDIOYldB 1-. a: passage wforw the .felectron .heamrinnwhichrwthe focusing. takes; place.- Thezrmember'll3.1-.ismonstructed ima :novel mannergtazvprovide the-ade-=:..;sired:distribution a of: rmagnetomotivegforce :within. the'ifocusing. eregionsidelined-z byzithetpassage fi-ias-willvbe:describedzinidetail:at adatter point 55.-in-.the specification. 'I heiniagnetomctive force 1.2 is supplied by three cylindrical permanentmagnets |1, l8 and i9 supported in circumferentially spaced relationaround the exterior of the anode cylinder I and retained in position bya pair of soft iron washers and 2| which are secured against axialmovement by a rolled edge 22 on the upper or outer end of the anodecylinder H and by a sleeve 23 which is spot welded to the exterior ofthe anode cylinder The magnets are retained in position by a pair ofretaining washers 24 and 25 having suitable openings for receiving theends of the permanent magnets and welded or otherwise secured to theopposed faces of the soft iron washers 20 and 2 The openings in thewashers 20 and 2| are dimensioned to receiving the exterior of the anodeH and to provide effectively a continuation of the pole pieces providedby the enlarged ends Hi and l5 of the focusing member |3. The gap in themagnetic material provided by the relatively low permeability stainlesssteel of the anode sleeve assists in the even distribution of a fluxproduced by I the three magnets and provides a circumferential tweenwashers 20 and 2| provided by the washer 2 la and the two air gaps.

As will be readily understood by those skilled in the artthe final anodeis usually operated at the same voltage as the wall coating of the bulband for the purpose of making electrical contact with such a coating(not shown) a plurality of circumferentially spaced spring tabs 23a aresecured to the end washer 20 of the anode and focusing assembly.

.In accordance with an important aspect of the present invention, thequality of the focusing obtained is greatly improved as compared withprior art focusing arrangements. The improvement results .from a numberof structural features which, though usable individually, are combinedin a relatively simple structure.

In accordance with one feature of the present invention, the passage I6through which the beam passes is defined by a continuous wall id,

as distinguished from a focusing gap. The wall is of such cross sectionthat the magnetomotive force impressed across the ends thereof bymagnets ll, l8 and I9, saturates the wall section and providessubstantial focusing flux within. the passage l6. The length of thesaturating section is also preferably in the order of three or moretimes the radius of the focusing passage. Both of these features assistin providing a focus ing field which is more uniform, and generallyparallel to the axis of the focusing passage.

The term saturated as used here does not mean complete saturation withthe resultant reduction 7 in permeability of the saturated section tounity but rather a saturation suflicient to reduce the permeability to avalue clearly below that associated with unsaturated magnetic materials.This means that the degree of saturation may be controlled'by varyingthe cross section of the lens wall and in this way to obtain anon-uniform distribution of magnetomotive force. along the .tudinal'axis of the lens).

surface of the lens structure in an axial direction. In the embodimentillustrated in Fig. 3, this nonuniform thickness of the saturated wallsection is obtained by machining into the member |3 a plurality ofcircumferential recesses 26 which extend into the member differentradial distances. In the particular construction shown, the wallthickness increases toward the central portion of the lens member(measured along the longi- This particular variation in cross section ofthe lens tends to provide a lens having negative spherical aberrationand by proper control of the amount of this aberration it can be made tojust compensate for the aberration of the electrostatic lens existing inthe gun structure and including, for example the lens provided bycylinders I and II which are normally operated at different voltages.

When the saturable wall section is formed in this mannen'the radialextending ribs or fins 28 provide a path for leakage flux from themagnets [1, I3 and I3 which does not afiect the focusing action of thelens and which has a relatively low reluctance circumferentially and arelatively high reluctance in a longitudinal direction. If the recessesare as deep or deeper than their axial width then the shieldingaccomplished by them is essentially complete and the localized leakageflux from the individual magnets does not disturb the uniformity of thefocusing field.

The non-magnetic gaps interposed between the pole pieces l4 and I5 andthe washers 20 and 2| respectively and provided by the anode cylinder Hin Fig. 3 assist materially in obtaining a uniform distribution ofmagnetomotive force across the entire circumference of the pole piecesl4 and I5. This combination of the non-magnetic gaps and the shieldingprovided by the ribs 23 render the action of the individual magnetsessentially as effective as a perfectly magnetized cylinder of permanentmagnet material. This is a substantial advantage, since it is verydiflicult to obtain a perfectly magnetized cylinder of permanent magnetmaterial andin addition such a structure is rather expensive.

In Figs. 4 and 5, I have shown a modified form of my invention which isin some respects easier to assemble. The focusing member 30 is ingeneral similar to the member |3 as shown in Fig. 3 but is difierentlyshaped at the ends in order to facilitate assembly with a pair ofwashers 3| and 32 having circular flanges 33 at the outer edges thereofand directed toward one another. The member 33 is formed with a flaredportion 34 providing a shoulder on which the washer 32 may rest, and atthe other end with a section 35 of reduced cross section which may bespun out over the upper surface of the washer 3| during assembly toclamp the washer 3| and 32 and the three permanent magnet members 36,3'|.and 38 together. As illustrated in Fig. 4, a suitable non-magneticgap between washers 3| and 32 and the parts 34 and 35 focusing member 30is provided by interposed washers 39 and 40 of suitable non-magneticmaterial such as stainless steel. The washers 3| and 32 and the parts 34and 35 of focusing member 3|] combine to provide the pole pieces of thefocusing structure. As apparent from an inspection of Figs. 4 andtheflanges 33 of the washers, 3| and 32. are shaped to retain the permanentmagnets in cir- .cumferential position. The assembly including thefocusing. member 39, thewashers 3| 1and32 and the permanent-magnetmembers 36, 31 and 38 is secured on the outer end of a second anodeorbrazed tothe face of the washer .32. 'be appreci'ated that thestructure just described "sired. For "example, the clamped together by astainless steel eyelet iltending circumferential flange 42 which iswelded It will operates in the same manner as the one described inconnection with Figs. 1, 2 and3, and

mayprovide a' more economical structure from a manufacturing point ofview.

In Fig.6, I have shown a still further modi- 5 fication of myinventionin which the lens structure- 43 corresponds generallyto the lensmemberl3 ofFig. 3. The lens structure instead of utilizing a saturablewall'section utilizesa stack of washers which are alternately ofmagnetic r an substantially non-magnetic material. Referring now to Fig;6; the assembly includes an annular stack including soft iron p'olepieces 44 -and-;'45 separated by a plurality of washers 46 "ofnonmagnetidmaterialsuch as stainless steel with'interpcsed washers l! ofhigh permeability iron. The washers 4t and 41 are substantially smallerouter diameter than the pole pieces 4'1 and t5 leaving room toaccommodate annular cylindrical permanent magnet 48 between the polepieces 14 and 45.

The assembly may be secured together in any suitable manner and attachedto the end of the final anode, if deassembly may rbe lustrated' at 49. y

it will be noted thatin' the modification in "Big. 6 that the washers ofstainless steel provide a path of relatively low permeance and the ironwashers provide a path of relatively high permeance.

Since the washers of low permeability are made progressively thinner to-Ward the center of the stack and the washers of] high permeability aremade progressively thicker'toward the center, it is apparent that themagnetic potential gradient is along the wall "defining the beam passagedecreases toward the longitudinal center of the lens while the magneticpotential gradient is substantially constant along the axis.

This is the same type of disturbance as was obtained by the saturablewall o f the. lens of Fig. 3 where the wall section is or greaterthickness near the longitudinal'center of Q the lens.

,InFig. 7, I have shown a still further modiffication to my inventionwhich corresponds generally to the modification shown in Fig. 3 andcorresponding parts have been. designated by the same referencenumerals. .In the construction shown in Fig. 7, the lens member 13includes annularpole pieces [4 and I5 and a generally cylindrical wallportion 50 extending between the pole pieces and having a varying crosssection'along its length to provide the desired distributionofmagnetomotive along the focusing passage. that. shown in Fig. 3primarily in the arrangement provided for obtaining a low reluctancepath. in a circumferential direction without efiec- "tively shortcircuiting the focusing passage in an axialor longitudinal direction. Inthis modi- The modification of Fig. 7 differs from fication, this pathis provided by a pair of annular or ring like members 5! and 52 of lowreluctance material, such as soft iron, interposed between the magnetsl1, l8 and l 9 and the focusing cylinder. These rings may to advantagebe supported from opposite sides of the nonmagnetic cylinder provided bythe stainless steel anode cylinder 5 l which provides a non-magnetic gaptherebetween. It is appreciated that the "rings- SI" and 52' are spaced"at "their ends-from the pole pieces l4. and 15 so as to provide s,

high :-r'eluctance path ina longitudinal direction. These ring membersprevent the localized magnetomotiveforce of the'three magnets fromdisturbing the uniformity of the focusing field in .a plane transverseto the axis of the focusing cylinder.

In the-foregoing description, a number "of modifications embodying myinvention have been described. It will be appreciated thatin' itsbroader aspects,- the invention involves the utilization of a physicalstructure between the pole l pieces ofa magnetic lens which {presents arelatively low' magnetic permeability as compared with unsaturated ironand which more specificallyprovidesa non-uniform and predeterminedpermeability along its length to give a spherical aberration free lensor a lens having a predetermined amount of special aberrationtocompensate for spherical aberration of any other lens which-acts on thebeam bein focused. "It has been found that for a spherical aberrationfreelens the area under the curve of magnetomotive force versus axialdistance along the lens defining the beam passage.

"While I have shown my invention applied to a straight electron 'gun,thatisa gun having no ion trap, it will be apparent that it maybe usedwith such guns with all of the advantages of controlled correctionofaberration that have been described in connection with the illustratedembodiment.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A magnetic-focusing structure for an electron beam comprising i agenerally cylindrical lens of magnetic material having a passage'therethrough, said lens including a generally annular pole piece ateach end thereof and. an intermediate hollow cylindrical section havinga varyingcross section along its length, and meanssurrounding said lensmember and impressing a magnetizing force across said pole pieces of astrength to produce saturationoi said cylindrical "section to produce anon-uniform magnetic potential gradient along the length of the innersurface of said hollow cylindrical section.

2. A'magnetic focusing structure for an electron bamcomprising agenerally cylindrical lens 'memberof magnetic material having a passagetherethrough, said member including a generally annular pole piece ateach end thereof and an intermediate hollow cylindrical section having avarying cross section along its length to produce a non uniform magneticpotential gradient along the length" of theinner surface of said hollowcylindrical section when a magnetomotive force Orsuflicient strength tosaturate said hollow "cylindrical section is impressed acrosssaid pole"pieces.

3. A magnetic focusing structure for an electron beam comprising agenerally cylindrical lens member of magnetic material having a passagetherethrough, said member includin a generally annular pole piece ateach end thereof and an intermediate hollow cylindrical section having across section substantially less than the cross section of said polepieces, and means surrounding'. said lens member and impressing a magnetizing force across said pole pieces of a strength to produce saturationof said intermediate hollow cylindrical section.

4. A magnetic focusing structure for an electron beam comprising agenerally cylindrical lens member of magnetic material having a focusingpassage therethrough, said member including a generally annular polepiece at each end thereof and an intermediate hollow cylindricalsection, a plurality of radially extending ribs of magnetic material onthe exterior of said cylindrical section and a plurality ofcircumferentially spaced permanent magnet members positioned outside ofsaid cylindrical section for impressing a magnetizing force across saidpole pieces.

5. A magnetic focusing structure for an electron beam comprising agenerally cylindrical lens member of magnetic material having a passagetherethrough, said member including a generally annular pole piece ateach end thereof and an intermediate hollow cylindrical section, a ringof magnetic material surrounding each of said pole pieces, non-magneticmeans interposed between the respective rings and pole pieces, aplurality of elongated permanent magnets extending between said ringsand for impressing a magnetizing force across said pole pieces.

6. A magnetic focusing structure for an electron beam comprising agenerally cylindrical lens member of magnetic material having a passagetherethrough, said member including a generally annular pole piece ateach end thereof and an intermediate hollow cylindrical section, a ringof magnetic material surrounding each ofsaidpole pieces, non-magneticmeans interposed between the respective rings and pole pieces, aplurality of elongated permanent magnets extending between said ringsand for impressing a inagnetizingforce across said pole pieces, saidhollow cylindrical section having a plurality of longitudinally spacedoutwardly extending ribs of magnetic material providing leakage pathsfor flux produced by said permanent magnets.

'7. A magnetic focusing structure for an electron beam comprising agenerally cylindrical lens member of magnetic material having acylindrical passage therethrough, said member including a generallyannular pole piece at each end thereof and an intermediate hollowcylindrical section having a cross section substantially less than thecross section of said pole pieces, the length of said cylindricalsection being in excess of three times the radius of said passage andmeans surrounding said lens member and impressing a magnetizing forceacross said pole pieces of a strength to produce saturation of saidintermediate hollow cylindrical section.

8. A magnetic focusing structure for an electron beam comprising agenerally cylindrical lens member of magnetic material having acylindrical focusing passage therethrough, said member including agenerally annular pole piece at each end thereof and an intermediatehollow cylindrical section having a predetermined non-uniform permeancealong its length to produce a non-uniform magnetic potential gradientalong the inner surface of said hollow cylindrical 'section when amagnetomotive force is impressed across said pole pieces.

9. A magnetic focusing structure for an electron beam comprising agenerally cylindrical lens having a focusing passage therethrough, saidlens including a generally annular pole piece of high permeance at eachend thereof and an intermediate hollow cylindrical section including astack of alternately high and low permeance rings of differingthicknesses to produce a nonuniform distribution along the length of theinner surface of said cylindrical section when a magnetomotive force isimpressed across said pole pieces.

10. In combination, an electron gun for producing a beam of electronsand including a cylindrical anode, a magnetic focusing lens supportedfrom said anode and having a cylindrical passage therethrough inalignment with the axis of said anode, said focusing lens including apair of annular pole pieces and a relatively thin Walled generallycylindrical portion interposed between said pole pieces and meansimpressing a magnetomotive force across said pole pieces of a magnitudeto produce substantial saturation of said wall portion.

11. A magnetic focusing structure for an electron beam comprising afocusing lens having a cylindrical passage therethrough in alignment,said focusing lens including a pair of annular pole pieces and arelatively thin walled generally cylindrical portion interposed betweensaid pole pieces, individual magnets supported in spaced circumferentialrelation and said lens and impressing a magnetomotive force across saidpole pieces of a magnitude to produce substantial saturation of saidthin walled portion and means interposed between said magnets and saidthin walled portion providing a low reluctance circumferential patharound said thin walled section and a high reluctance path between saidpole pieces.

12. In combination, an electron gun for producing a beam of electronsand including a cylindrical anode of non-magnetic material, a magneticlens within said anode member including a pair of annular pole pieceshaving an outer surface conforming in size and shape to the interiorsurface of said anode, a pair of annular washers of magnetic materialsurrounding said anode and located opposite said pole pieces in an axialdirection, and permanent magnet means positioned on the outside of saidanode and extending between said washers.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,159,534 Ruska May 23,1939 2,188,579 Schlesinger Jan. 30,1940 2,212,206 Holst et al Aug. 20, 1940 2,259,531 Miller et al Oct. 21,1941 2,305,761 Borries et a1 Dec. 22, 1942 2,418,349 Hillier et al Apr.1, 1947 2,503,173 Reisner Apr, 4, 1950

